1. Field of the Invention
The present invention relates to an injection molding process for manufacturing injection molded ceramic articles having excellent in quality and properties, and to molds to be used therefor.
2. Description of the Prior Art
Since silicon ceramics, such as silicon nitride, silicon carbide, SIALON, or the like, are more stable and less susceptible to oxidation corrosion or deformation at high temperatures than metals, active research has been conducted recently on utilization of silicon ceramics as engine parts. For example, radial turbine rotors made of these ceramic materials are lighter and superior excellent in thermal efficiency, thus allowing operating temperatures of engines to be raised, as compared with rotors made of metals. Accordingly, silicon ceramics have been drawing attention for use as a turbo charger rotor, gas turbine rotor, etc. for automobiles.
Since such a turbine rotor has intricate three-dimensional shaped blades, naturally it is very difficult to finish such a rotor by grinding sintered solid materials of simple shapes, for example, dense silicon nitride or silicon carbide sintered bodies shaped as a circular cylinder, square cylinder or the like, into a desired shape.
As processes for molding ceramics, the following are well known: a plastic molding process, such as extrusion molding or the like, wherein plasticity of molding materials is utilized; a slip cast molding process wherein a slip, namely, an aqueous suspension of ceramic starting material powder, is poured into a mold; a dry pressure molding process wherein a prepared powder is loaded into a mold and pressed; and the like. Other than the above, injection molding processes that have been extensively employed in molding of plastics have recently begun to be applied in molding ceramics into irregular or intricate shapes as well.
The injection molding processes have been performed mainly for thermoplastic resins in plastic molding, wherein heat-fluidized plastic materials are pressurized by a plunger or the like, pushed into a chilled metal mold and solidified by cooling into an integral, molded body. In such injection molding processes, various improvements have been made through many years of research in the plastics industry.
However, in the ceramics industry, it has heretofore been considered that qualities and properties of final molded products mainly depend upon starting material fine powders. Therefore, it is the present situation that extensive technical developments have been achieved in preparation of starting material fine powders, while research and development of molding processes have fallen behind. Recently, the molding processes have been found to influence largely upon qualities, etc. of molded products, so that the molding processes are now being reviewed. Particularly, recently injection molding processes began to be applied in ceramic molding and, therefore, injection molding machines, metal molds or the like are still at the stage that many improvements are required.
In the injection molding processes of ceramics, since conventional ceramic material fine powders, per se, different from plastics, have no plasticity, there have been employed molding materials, such as pellets, plasticized by admixing a starting material fine powder with a thermoplastic resin, or a molding material (kneaded material or pug) obtained by adding water as a plasticizing medium. Such processes have been proposed by the assignee of the present application in Japanese Patent Application Laid-open No. 64-24,707. Namely, injection molding processes comprise the steps of: mixing a ceramic powder with an organic binder comprising a thermoplastic resin, such as polyethylene, polystyrene or the like, a plasticizer, a dispersant, wax, etc.; plasticizing by heating the mixed material; and injecting the plasticized material into a metal mold. Alternatively, there are also known injection molding processes comprising the steps of: mixing a ceramic power with mainly water as a plasticizing medium and an organic binder as a plasticizer; plasticizing by cooling the resulting mixture; and injecting the plasticized material into a metal mold. The thus obtained molded bodies are heated to burn organic binder and then fired to provide ceramic sintered products. According to the above molding processes, molded bodies such as intricate parts can be obtained rapidly with high accuracy by a single operation at a low cost, which intricate parts would otherwise require considerable time and money to produce.
However, the inclusion of air bubbles or non-homogeneity maybe induced in the molding materials during injection from an injection molding machine to a metal mold, since these molding materials are low in fluidity as compared with thermoplastic resins or cannot be sufficiently fluidized by heating. In particular, with regard to molding materials using mainly water as a plasticizing medium, as shown in the above described Japanese Patent Application Laid-open No. 64-24,707, of which physical properties, etc. have not been elucidated yet, development of conditions, etc. to be applied in injection molding processes has been expected.
Meanwhile, as for the temperature of the metal molds in conventional injection molding processes, it is usually equalized throughout the mold from its gate up to the endmost portion. However, when the temperature of the metal mold is equalized, the molding material differs in temperature between near the gate portion and the endmost portion of the metal mold during injection molding, resulting in cracks, deformation or the like in sintered products obtained by firing molded bodies, thus providing sintered products with low and uneven dimensional accuracy, strength or the like. Therefore, heretofore homogeneous sintered products have not been able to be obtained.